The relevant area of expertise has long been engaged in determining imaging rules. U.S. Pat. Nos. 5,963,612 and 5,963,613 are examples of this. The imaging rule is normally determined with the aid of a “calibration phantom”. Such a calibration phantom has pre-determined structural elements that are reflected in certain structures in an X-ray image of the calibration phantom and the imaging rule can be deduced from these imaged structures. U.S. Pat. No. 6,044,132 describes an example of a calibration phantom. A different calibration phantom is described in the article by N. Strobel, B. Heigl, T. Brenner, O. Schutz, M. Mitschke, K. Wiesent, T. Mertelmeier, entitled “Improving 3D image quality of X-ray C-arm imaging systems by using properly designed posed determination systems for calibrating the projection geometry, Medical Imaging 2003”, and was published in the year 2003 in the book entitled “Physics of Medical Imaging” by Yaffe, Martin J., Antonuk, Larry E., Proceedings of the SPIE, Vol. 5030, pages 943 to 954.
A precondition to enable the imaging rule to be determined with the aid of an image of a calibration phantom is normally that a minimum amount of the structural elements can be imaged in the X-ray image. If the calibration phantom is placed on the patient table, this means that the determination of an imaging rule at any positions of the imaged components as required is not possible. At some positions, the amount of the calibration phantom that can be recorded is so small that a determination of an imaging rule is not possible. This problem is shown sharply with the method described in DE 10 2007 026 115.4, published after the date of filing of this application. The object of this method is to generate a 3D reconstruction of a particularly large body, which cannot be recorded by a single projection. To solve the problem, at least two projections (X-ray images), each of which records part of the body, are taken at each position of an X-ray C-arm, but, overall, each structure of the body is recorded on one of the two projections. DE 10 2007 026 115.4 is concerned with the problem that the real sources Q1 and Q2 do not coincide with both projections. After filtering with the aid of a virtual detector, the imaging rules, which have lead to both projections (i.e. both X-ray images), are used reversed and individually in a filtered back-projection to generate the 3D reconstruction. With the method from DE 10 2007 026 115.4, it is therefore necessary to determine the imaging rule for both positions of the imaging components at which both X-ray images were taken. Because these positions are chosen so that the particularly large body is imaged in the border area in each case and the border area is covered anyway, the X-ray images are individually recorded, i.e. each on its own, from a particularly unusual perspective. If a calibration phantom is placed on the patient table, this is only approximately half covered in each case. If the calibration phantom from the article by Strobel et al. is used, this would make the determination of the imaging rule impossible on the basis of a single X-ray image alone.